By adding a small amount of comonomer into PTFE homopolymer, it is possible to modify the PTFE polymer, which does not allow melt molding but allows formation of fibrils, so as to favor paste extrusion. The polymer prepared in this way is called modified PTFE so as to be distinguished from copolymer of tetrafluoroethylene (TFE) which allows melt molding.
For the paste extrusion molding, in order to improve the productivity, the tendency is to increase the RR. RR is represented by the ratio S2:S1, where S1 represents the cross-sectional area at the outlet of the die of the extruder and S2 represents the cross-sectional area of the cylinder that supplies the extrusion powder. Consequently, there is a demand for development of polymers which allow extrusion at higher RR, and which have molding with good appearance and high strength. Usually, as RR is increased, the extrusion pressure rises, and the extruded molding develops a waviness, rough surface, cracks, or breakage. Consequently, it becomes impossible to obtain normal moldings. The reason is believed to be as follows: When PTFE is extruded, the primary grains of PTFE receive shear, leading to the formation of fibrils. At the same time, orientation takes place in the extrusion direction of the primary grains, and friction takes place among grains. The fibril formation and friction appear as the extrusion pressure. Consequently, the extrusion pressure depends on the degree of fibril formation and the friction force. Under a high RR, these are promoted, leading to an increase in the extrusion pressure and poor quality of the molding. In order to improve the extrusion property under high RR, various techniques have been proposed.
In the polymerization method of PTFE disclosed in Japanese Kokoku Patent No. Sho 37 [1962]-4643, before 70% of the prescribed amount of TFE is consumed, a modifier is added into the polymerization system. Examples of the modifiers include hexafluoropropylene (HFP) and other perfluoroalkyltrifluoroethylenes, and methanol and other chain transfer agents. By the addition of a modifier into PTFE, the crystallinity of the polymer can be decreased, and the formation of fibrils can be suppressed. Consequently, the addition of modifier into PTFE can suppress excessive rise of the extrusion pressure, and to alleviate the problem of poor moldings.
The purpose of the method proposed in U.S. Pat. No. 4,792,594 is identical to that in the aforementioned approach disclosed in Japanese Kokoku Pat. No. Sho 37 [1962]-4643. The modifier used in this method is perfluorobutyl ethylene (PFBE). In the method disclosed in Japanese Kokoku Pat. No. Sho 56 [1981]-26242, at the time point when the reaction has been carried out for 70-85%, chlorotrifluoroethylene (CTFE) is added. Due to CTFE, a shell modification is produced for the portion near the surface of the primary grains. Consequently, a lower extrusion pressure is displayed, and the extrusion property at high RR is excellent for the obtained fine powder.
By using the methods disclosed in Japanese Kokoku Pat. No. Sho 37 [1962]-4643 and U.S. Pat. No. 4,792,594, the paste extrusion property of the PTFE fine powder can be improved. However, in order to improve the productivity further, there is a demand for the development of PTFE fine powder that allows molding at even higher RR. For the fine powder prepared according to the method disclosed in Japanese Kokoku Pat. No. Sho 56 [1981]-26242, the extrusion property at high RR is better than that of the fine powders prepared using the aforementioned two methods. However, the fine powder prepared in this method has a high thermal degradation index (TDI), that is, a poor heat resistance. The TDI is an index derived from the difference in density of moldings prepared with different baking times. Larger values of the thermal degradation index correspond to larger differences in density. This indicates that the polymer molecular chains are severed by heat, leading to a decrease in the molecular weight.
The fine powder for high-RR extrusion can be used to form electrical cable coating and fine tubes, which can be used in automobiles, airplanes, precision machines, etc., which require higher quality. In particular, cases of use of the fine powder in manufacturing peripheral parts for engines of automobiles are increasing, and the requirement for heat resistance is very strict for the electrical cables and tubes used in these cases. Consequently, in these fields, there is a demand for the development of fine powder with high heat resistance. For example, the CTFE modified fine powder disclosed in Japanese Kokoku Pat. No. Sho 56 [1981]-26242 exhibits a TDI in the range of 30-50. On the other hand, the modified fine powders disclosed in Japanese Kokoku Pat. No. Sho 37 [1962]-4643 and U.S. Pat. No. 4,792,594 exhibit a TDI in the range of 0-20.
The purpose of this invention is to solve the aforementioned problems of the conventional methods by providing a method of manufacturing PTFE fine powder with high extrusion property at high RR and with excellent thermal stability.